Various vehicle steering systems, which are referred to as a so-called steer by wire system, have been recently developed toward practical use. In conventional vehicles, a steering wheel and a steered wheel are generally and mechanically connected together through a mechanism such as a rack and pinion. In the steer by wire system, however, such mechanical connection is not provided, and a wheel is actually turned in the following way: a. steering angle suitable for a traveling state of a vehicle is determined by detecting with a sensor an input from a driver, such as, for example, torque, a steering angle, or the like, also in combination with information from other vehicle sensors; and the steering angle command is sent to a steering actuator.
When the steer by wire system is to be adopted, a connection mechanism by which a steering wheel and a steered wheel are mechanically connected together, or a similar mechanism, which is a mechanism for so-called fail-safe, is generally prepared separately from the steer by wire system, in order to secure steering performance if the steer by wire system is in a failure state. As such a mechanism, a steering device including, for example, a planet gear clutch is known (see Patent Document 1).